This invention relates to convergence apparatus for color television picture tubes and particularly to such apparatus for color television picture tubes having a plurality of electron guns arranged in a single plane, commonly referred to as "in-line" gun structure.
Color television receivers employing picture tubes of the multi-gun type have been known for many years. The most popular has been the three gun shadow mask tube in which three triangularly-disposed-electron-emitting guns, located in the neck of the picture tube, produce respective streams of electrons directed toward a target consisting of a mosaic of different colored-light-emitting phosphor deposits on the inner surface of the transparent picture tube face. A perforate shadow mask is mounted close to the screen and shields the phosphor elements from electrons emanating from any but their associated one of the electron guns. Conventional deflection apparatus vertically and horizontally scan the electron beams across the phosphor target.
Because of the different attitudes of the electron beam paths in the yoke deflection field, tolerances in gun and tube manufacture and because the guns are not located at the center of curvature of the screen, various misconvergence and purity errors occur. Static errors, that is, errors in the center of the tube under free fall (no deflection) conditions may be corrected with the help of external magnets and properly designed gun elements. Dynamic errors occur during deflection of the beams. Some of these errors may be compensated in the yoke design. In general, however, both static and dynamic beam correction devices are used to maintain the electron beams in correct relationship with respect to the picture tube mask and phosphor deposits. Many of these problems may be minimized by adjusting the design of the mask and phosphor mosaic target of the picture tube to the yoke.
Recently the "in-line" gun type of picture tube has become popular. In this tube the three electron guns are aligned in a horizontal plane. When used in conjunction with a vertically striped phosphor screen and a shadow mask with vertical slots, the problems of dynamic convergence of the beams are greatly simplified. Indeed, such a simplification is possible that the expensive and cumbersome dynamic convergence circuitry (required for delta gun arrangements) may be obviated. There are such tubes currently available which have yokes permanently cemented in place and which do not need dynamic convergence.
Further developments have been directed toward achieving better gun performance especially in the area of spot size, which is perceived by the viewer as sharper focus. One recently introduced electron gun arrangement uses extra elements for developing electron beams having desirable spot size characteristics under a wide range of beam intensities. It is referred to as an extended focus lens (EFL) gun and has one or more relatively high potential focusing elements, such as the gun described in U.S. Pat. No. 3,895,253 dated July 15, 1975.
In a delta gun arrangement, all guns are axially inclined to aid free fall convergence at the center of the shadow mask. In an in-line arrangement only the outer electron guns are canted to assist in the free fall convergence of the three beams. In both arrangements, external magnetic apparatus is still required to make minor adjustments necessitated by normal tolerances. In the delta arrangements, a magnet is used to move the center beam (usually blue) laterally to insure that a common static convergence point is obtained. In an in-line arrangement, a static magnetic device is provided for moving all three of the beams in a horizontal direction for producing purity, that is, assuring that the different colored-light-emitting phosphors are only impinged by electrons from their corresponding guns.
In an EFL type in-line gun, positioning of these external magnetic corrector devices (the static/purity device) is critical if the spot size advantages are to be optimized. As with delta gun arrangements the position of the corrector structures on the neck of the picture tube is determined by measuring from a gun element or a portion of the picture tube. Unfortunately, performance can be adversely affected by even minor errors in positioning.